HV Circuit-Breaker Switching Duties

Out-of-Phase Switching Duty for Circuit-Breakers

Out-of-phase switching is one of the most severe circuit-breaker duties from the point of view of transient recovery voltage. The current to be interrupted is normally much lower than the rated short-circuit breaking current, but the recovery voltage across the open contacts can be very high, because both sides of the circuit-breaker remain energised by active sources. This note explains what the duty means, where it occurs, how it differs from ordinary fault-current interruption, and how IEC and IEEE circuit-breaker standards treat it.

Reading time ≈ 18 min · Meaning, standard references & engineering interpretation

Out-of-phase switching is one of the most onerous interrupting duties a high-voltage circuit-breaker can face, but it is onerous in an unusual way. The current is often modest — well below the rated short-circuit breaking current — yet the voltage that appears across the open contacts after interruption can be far higher than in an ordinary fault, because there is a live, independently driven source on each side of the circuit-breaker. It is therefore a recovery-voltage duty first and a current duty second.

This page explains what out-of-phase switching means, where it can credibly occur, how it differs from short-circuit interruption, the simplified relationships behind the out-of-phase current and recovery voltage, and how the duty is assigned and tested under the IEC and IEEE circuit-breaker standards — including the special case of generator circuit-breakers.

Abbreviations used on this page

Out-of-phaseInterruption between two unsynchronised live systems

CBCircuit-breaker

GCBGenerator circuit-breaker

TRVTransient recovery voltage

RVRecovery voltage

RRRVRate of rise of recovery voltage

FPTCFirst-pole-to-clear (factor)

\(k_\text{pp}\)First-pole-to-clear factor

SLFShort-line fault

HV / ACHigh voltage / alternating current

\(I_\text{sc}\)Rated short-circuit breaking current

\(I_\text{oop}\)Out-of-phase current

\(U_\text{max}\)Rated maximum voltage of the circuit-breaker

\(X_\text{eq}\)Equivalent reactance between the two sources

\(Z_\text{eq}\)Equivalent surge impedance seen by the circuit-breaker

\(X''_d\)Generator subtransient reactance

\(U_\text{RV}\)Recovery voltage across the circuit-breaker

\(V_1,\ V_2\)Voltage phasors on each side of the circuit-breaker

\(X_\text{tr}\)Transformer short-circuit reactance

\(X_\text{sys}\)Equivalent system reactance

\(du/dt\)Rate of rise of recovery voltage

\(di_\text{oop}/dt\)Rate of change of out-of-phase current

\(\delta\)Phase-angle difference between the two systems

EMTElectromagnetic transient (study)

IECInternational Electrotechnical Commission

IEEEInstitute of Electrical and Electronics Engineers

Key idea

Two live sources, not a source and a fault. Out-of-phase switching opens a circuit-breaker between two energised systems that are not in synchronism. The current is usually moderate, but the recovery voltage can be very high because both sides stay energised.
An assigned duty, not a default rating. Out-of-phase capability is application-specific. Do not assume a circuit-breaker has it unless it is stated in the specification, nameplate, type-test evidence or manufacturer documentation.
Typically 25% of \(I_\text{sc}\) — but check the TRV. Where assigned, the out-of-phase breaking current is normally 25% of the rated short-circuit breaking current, yet the associated TRV peak and RRRV must be confirmed, not just the current.
Pick the right standard. Use IEC 62271-100 (with 62271-101 for synthetic testing) or IEEE C37.04 with C37.09 for general circuit-breakers; use IEC/IEEE 62271-37-013 for generator circuit-breakers.

Key terms used on this page

01Out-of-phase switching

Interrupting current between two energised parts of the power system that are not in synchronism.

02Loss of synchronism

Two sources that should rotate together drift apart in phase angle, so a large angle difference appears across the circuit-breaker.

03Phase opposition

The most severe out-of-phase condition, with the angle difference approaching 180°.

04Recovery voltage

The voltage that appears across the open contacts once the current has been interrupted.

05Transient recovery voltage

The transient part of the recovery voltage immediately after current zero, before it settles to the power-frequency value.

06RRRV

Rate of rise of recovery voltage — the initial slope of the TRV, important for re-ignition.

07First-pole-to-clear factor

The factor by which the voltage on the first pole to interrupt exceeds the normal phase voltage.

08Out-of-phase current

The current driven through the circuit-breaker by the voltage-angle difference between the two connected systems.

09Short-line fault

A fault a short distance down a line; a high-RRRV duty, used here for contrast with out-of-phase.

10Effectively earthed

A neutral-earthing arrangement that limits temporary overvoltage and sets the lower first-pole-to-clear factor.

11Generator circuit-breaker

A circuit-breaker between a generator and its step-up transformer, covered by IEC/IEEE 62271-37-013.

12Equivalent surge impedance

The characteristic impedance seen by the circuit-breaker, which governs the initial line-side TRV slope.

Section 1

What out-of-phase switching means

Out-of-phase switching occurs when a circuit-breaker is required to open a connection between two parts of a power system that are not in synchronism.

In a normal synchronised system, the voltage phasors on both sides of a closed circuit-breaker rotate at almost the same frequency, with only a small phase-angle difference between them. During severe system disturbances the two sides may drift apart, so the voltage on one side of the circuit-breaker has a markedly different phase angle from the voltage on the other side.

The out-of-phase condition becomes severe when the phase-angle difference approaches full phase opposition, meaning approximately 180°. In that condition the instantaneous voltage across the open circuit-breaker contacts can approach the sum of the two system voltages.

The phase-angle difference between the two systems is often written \(\delta\). When \(\delta\) is small, the voltage difference between the two systems is limited; as \(\delta\) approaches 180°, the voltage across the open circuit-breaker contacts can approach the sum of the two source voltages.

In simple terms

A normal circuit-breaker opens between a source and a fault or load. An out-of-phase circuit-breaker opens between two live sources that are not synchronised. The current may be moderate, but the voltage appearing across the circuit-breaker after interruption may be very high. This is why out-of-phase switching is mainly a recovery-voltage duty rather than a maximum-current duty.

Standards referenced on this page

The duty is treated in both the IEC and the IEEE circuit-breaker frameworks. The main standards normally referenced are summarised below; for generator circuit-breakers, the dedicated generator standard should be used instead of relying only on the general circuit-breaker standard.

Table 1 — Standards commonly referenced for out-of-phase switching duty.

Standard	Role / Scope
IEC 62271-100	General high-voltage AC circuit-breakers
IEC 62271-101	Synthetic testing for AC circuit-breaker making and breaking tests
IEEE C37.04	Ratings and requirements for AC high-voltage circuit-breakers
IEEE C37.09	Test procedures for AC high-voltage circuit-breakers
IEC/IEEE 62271-37-013	AC generator circuit-breakers

Always check the applicable edition

Standard clause numbers, test duties and terminology may change between editions. The applicable edition stated in the project specification and the manufacturer’s type-test report should always be checked before using these values in procurement or compliance assessment.

Section 2

Why this duty is not always a standard rating

Out-of-phase switching is not required for every general-purpose circuit-breaker application. Many circuit-breakers will never intentionally be used to separate two live systems that are significantly out of synchronism. For this reason, out-of-phase switching capability is normally treated as an assigned or application-specific capability rather than a mandatory standard rating for all circuit-breakers.

Do not assume the capability is there

A circuit-breaker should not automatically be assumed to have out-of-phase switching capability unless the duty is stated in the specification, the nameplate data, the type-test evidence or the manufacturer’s technical documentation. For projects where out-of-phase separation is credible, the engineer should explicitly specify the required out-of-phase switching duty.

When to specify this duty

Specify out-of-phase switching capability when the circuit-breaker can separate two live sources — two grid areas, a bus-coupler, a tie-line, a generator connection, or a system-restoration path — where synchronism may not be guaranteed.

In one line

Out-of-phase switching is generally a moderate-current, high-recovery-voltage duty — and it is an assigned capability that has to be specified and proven, not a property every circuit-breaker carries by default.

Section 3

Typical situations where out-of-phase switching can occur

Out-of-phase switching may occur in several practical situations. One common case is separation between two parts of a transmission system after a major disturbance: if the two areas lose synchronism, the circuit-breaker connecting them may need to interrupt the out-of-phase current.

Another important case is generator switching. A generator can fall out of step with the grid following a fault, delayed clearing, loss of excitation, incorrect synchronising, system instability or protection maloperation. If the generator is separated from the power system under these conditions, the circuit-breaker may experience an out-of-phase duty. The duty can also arise during system restoration, islanding, reclosing onto an unsynchronised system, or operation of tie-line circuit-breakers during unstable power swings.

Typical examples include:

Tie-line circuit-breakers between two grid areas.
Bus-section or bus-coupler circuit-breakers connecting two live sources.
Circuit-breakers on radial or weak transmission systems.
Generator circuit-breakers.
HV circuit-breakers on the transformer high-voltage side of a generator transformer.
Circuit-breakers exposed to synchronising or mis-synchronising duty.
Circuit-breakers used during system restoration or island reconnection.

Section 4

Out-of-phase switching versus fault-current breaking

Out-of-phase switching should not be confused with ordinary short-circuit interruption. In a short-circuit interruption, the circuit-breaker interrupts a fault current; after current interruption, one side may remain energised by the source, while the faulted-side voltage usually collapses or decays depending on the circuit configuration.

In an out-of-phase interruption, both sides of the circuit-breaker may remain energised. The voltage on each side continues to be driven by its own source, so the recovery voltage across the open circuit-breaker can be the vector difference between two live source voltages. This is what makes the out-of-phase TRV special.

Table 2 — Difference between out-of-phase interruption and ordinary short-circuit interruption.

Aspect	Short-Circuit Interruption	Out-of-Phase Interruption
Interrupted current	Up to rated short-circuit breaking current	Normally lower than the rated short-circuit breaking current
After current zero	Faulted side voltage typically collapses or decays	Both sides remain energised by active sources
Recovery voltage	Largely set by the source side	Vector difference of two live source voltages — can be very high
TRV peak	High, but bounded by one-sided source	Can be the highest among several switching duties
RRRV	Can be very high (e.g. short-line fault)	Normally moderate by comparison
Governing stress	Current interruption	Dielectric withstand after current zero

Useful engineering summary

Out-of-phase switching is often a moderate-current, high-recovery-voltage duty: the current is normally lower than the rated short-circuit breaking current, the TRV peak can be very high, the rate of rise of recovery voltage is normally moderate compared with some short-line-fault duties, and the circuit-breaker must withstand the recovery voltage between two active systems.

Not the same as a short-line or long-line fault

Out-of-phase switching should not be confused with short-line or long-line fault interruption. SLF and LLF are line-fault TRV duties dominated by travelling waves; out-of-phase switching is a two-source recovery-voltage duty, in which both sides of the circuit-breaker remain energised. See Short-Line and Long-Line Faults ↗.

Section 5

Out-of-phase current

The out-of-phase current is the current flowing through the circuit-breaker because of the voltage-angle difference between the two connected systems. It depends on the voltage magnitude on both sides of the circuit-breaker, the phase-angle difference between the two systems, the equivalent source impedance on each side, the line, transformer or generator reactance between the equivalent sources, the location of the electrical equilibrium point, and whether the duty is between two systems or between a generator and the grid.

A simplified concept, treating the path between the two sources as predominantly reactive, is:

\[ I_\text{oop} \approx \frac{\left|\,V_1 - V_2\,\right|}{X_\text{eq}} \]

\(I_\text{oop}\): out-of-phase current
\(V_1,\ V_2\): voltage phasors on the two sides of the circuit-breaker
\(X_\text{eq}\): equivalent reactance between the two sources

A simplified explanation only

This expression is only a simplified explanation. A detailed assessment should use an appropriate network model, because resistance, transformer impedance, generator subtransient reactance, line propagation and system configuration can all affect the result. As the phase-angle difference increases, the voltage difference \(\left|V_1 - V_2\right|\) increases; at full phase opposition the voltage difference is at its maximum, and so is the out-of-phase current for a given \(X_\text{eq}\).

Section 6

Out-of-phase recovery voltage and why the TRV peak is high

After the circuit-breaker interrupts the out-of-phase current, the voltage across the circuit-breaker contacts is determined by the difference between the source-side voltage and the opposite-side voltage. In ordinary fault clearing, the load-side or fault-side voltage component may decay. In out-of-phase switching, the opposite side remains energised, so the recovery voltage does not simply decay to zero on one side — both sides contribute to the voltage across the circuit-breaker. This is why the out-of-phase TRV can have the highest peak value among several switching duties.

The key reason the TRV peak can be high is that both sides of the circuit-breaker remain energised. If the two source voltages are out of phase, the voltage across the contacts after interruption can become much larger than the voltage from one side to ground. At full phase opposition, the voltage across the circuit-breaker can approach the sum of the two phase-to-ground voltages.

The important parameters that describe the duty are:

Power-frequency recovery voltage.
TRV peak value.
RRRV — the rate of rise of recovery voltage.
First-pole-to-clear factor.
Equivalent surge impedance, especially for line separation.
Natural frequencies of the networks on both sides.

Why this matters for the circuit-breaker

Because the stress is dielectric rather than thermal, out-of-phase duty is best described as: lower current than short-circuit breaking; higher recovery voltage than many ordinary fault-clearing cases; moderate rate of rise compared with some short-line-fault cases; and severe dielectric stress immediately after current zero.

Section 7

First-pole-to-clear and out-of-phase factors

The first-pole-to-clear factor represents the voltage stress on the first pole of a three-phase circuit-breaker to interrupt the current. For ordinary terminal-fault duty its value is 1.3 for effectively earthed neutral systems and 1.5 for non-effectively earthed neutral systems. Out-of-phase breaking is more severe, so the prospective recovery voltage is built on substantially higher out-of-phase factors.

Table 3 — Recovery-voltage factors used for out-of-phase duty and terminal-fault comparison.

Neutral Earthing	Out-of-Phase Factor	Terminal-Fault Factor (for Comparison)
Effectively earthed neutral	2.0	1.3
Non-effectively earthed neutral	2.5	1.5

For terminal-fault interruption, the first-pole-to-clear factor is commonly represented by values such as 1.3 for effectively earthed systems and 1.5 for non-effectively earthed systems. For out-of-phase switching, the recovery-voltage duty is represented by higher out-of-phase factors, commonly 2.0 for grounded / effectively earthed systems and 2.5 for ungrounded / non-effectively earthed systems. These values should not be confused with the ordinary terminal-fault first-pole-to-clear factors, and they should not be treated as ordinary continuous ratings of the circuit-breaker; the higher value for non-effectively earthed systems reflects the more severe voltage condition that may appear across the first clearing pole.

Section 8

IEC reference for general AC circuit-breakers

For general high-voltage AC circuit-breakers, the relevant IEC standard is IEC 62271-100 — High-voltage switchgear and controlgear — Part 100: Alternating-current circuit-breakers. This standard applies to three-phase AC circuit-breakers above 1 kV for operation at 50 Hz and/or 60 Hz.

For out-of-phase duty, IEC 62271-100 includes out-of-phase making and breaking requirements and associated test duties, and defines prospective TRV values for these tests. Where synthetic testing is used to demonstrate the duty, the techniques are covered by IEC 62271-101.

A commonly used IEC-based engineering rule is that, where assigned, the rated out-of-phase breaking current is normally 25% of the rated short-circuit breaking current unless otherwise specified. Therefore, for a circuit-breaker with a rated short-circuit breaking current of:

\[ I_\text{sc} = 40\ \text{kA} \qquad\Longrightarrow\qquad I_\text{oop} = 0.25 \times 40\ \text{kA} = 10\ \text{kA} \]

\(I_\text{sc}\): rated short-circuit breaking current
\(I_\text{oop}\): typical assigned out-of-phase breaking current (25% of \(I_\text{sc}\))

Current capability is not the whole story

This does not mean the circuit-breaker is suitable for any out-of-phase duty. The associated TRV — its peak, its rate of rise and the recovery-voltage level — must also be checked against the circuit-breaker’s tested capability.

Use the 25% value carefully

The 25% figure is the commonly assigned or preferred out-of-phase breaking current for general circuit-breaker applications — not a universal rule for every circuit-breaker or every system condition. Generator circuit-breakers and project-specific duties may require a separate assessment.

Section 9

IEEE reference for general AC circuit-breakers

For IEEE-based specifications, the relevant standards are IEEE C37.04 — IEEE Standard for Ratings and Requirements for AC High-Voltage Circuit Breakers with Rated Maximum Voltage Above 1000 V and IEEE C37.09 — IEEE Standard Test Procedures for AC High-Voltage Circuit Breakers with Rated Maximum Voltage Above 1000 V. IEEE C37.04 is the rating-and-requirement standard; IEEE C37.09 is the test-procedure standard.

The IEEE C37.04-1999 text for out-of-phase switching current capability states that this duty is required only for certain circuit-breaker applications and is not considered necessary as a standard rating for general-purpose circuit-breakers. Where an assigned out-of-phase switching current rating is specified, the preferred value is 25% of the rated symmetrical short-circuit current unless otherwise specified.

The same IEEE text defines the rated power-frequency out-of-phase recovery voltage as follows, for grounded and ungrounded systems respectively:

\[ U_\text{RV} = \frac{2\,U_\text{max}}{\sqrt{3}}\quad\text{(grounded)} \qquad\qquad U_\text{RV} = \frac{2.5\,U_\text{max}}{\sqrt{3}}\quad\text{(ungrounded)} \]

\(U_\text{RV}\): rated power-frequency out-of-phase recovery voltage
\(U_\text{max}\): rated maximum voltage of the circuit-breaker
factor 2: severe voltage difference possible between two grounded systems in out-of-phase operation
factor 2.5: more severe condition for ungrounded systems

Recovery-voltage example

For a circuit-breaker with a rated maximum voltage \(U_\text{max} = 420\ \text{kV}\) on an effectively earthed (grounded) system, the rated power-frequency out-of-phase recovery voltage is \(U_\text{RV} = 2 \times 420/\sqrt{3} \approx 485\ \text{kV}\); on an ungrounded system the corresponding value would be \(2.5 \times 420/\sqrt{3} \approx 606\ \text{kV}\). This value should then be compared with the circuit-breaker’s tested recovery-voltage / TRV capability.

The interrupting time for out-of-phase switching may also be permitted to exceed the rated interrupting time. Based on the IEEE C37.04-1999 text, the permitted increase is 50% for circuit-breakers rated five cycles or more, and one cycle for circuit-breakers rated three cycles or fewer.

Check the contract edition

For current projects, the exact edition of IEEE C37.04 and IEEE C37.09 specified in the contract should always be checked, because clause numbering and detailed wording can change between editions.

The role of IEEE C37.09

Do not use IEEE C37.09 as the primary source for defining the assigned out-of-phase current rating: use IEEE C37.04 for the rating basis and IEEE C37.09 for the corresponding test procedure. IEEE C37.09 should be referenced when discussing test procedures, and should not be used as the only source for the rating value unless the relevant clause has been checked in the applicable edition. The rating structure belongs mainly to IEEE C37.04, while IEEE C37.09 describes how the tests are performed to verify the assigned ratings and associated capabilities.

Careful report wording

“The circuit-breaker out-of-phase switching rating should be specified in accordance with IEEE C37.04, and the corresponding test procedure should be in accordance with IEEE C37.09.” This wording avoids incorrectly placing the rating definition in the test-procedure standard.

Section 10

Generator circuit-breaker applications

Generator circuit-breakers require special attention. For generator circuit-breakers, the correct standard is IEC/IEEE 62271-37-013 — High-voltage switchgear and controlgear — Part 37-013: Alternating-current generator circuit-breakers. This standard is applicable to three-phase AC generator circuit-breakers for indoor or outdoor installation, for operation at 50 Hz and 60 Hz, on systems above 1 kV. In practice these circuit-breakers are applied at generator-terminal voltages — commonly up to a few tens of kV (often quoted as up to around 38 kV) — rather than at transmission voltages.

Generator circuit-breakers are exposed to special duties because the generator and the power system can both contribute to the recovery voltage. The generator subtransient and transient reactances, the transformer reactance, the excitation condition and the system reactance all strongly influence the out-of-phase duty. For generator circuit-breakers, out-of-phase conditions may occur during loss of synchronism, generator instability following delayed fault clearing, mis-synchronisation, power-plant tripping, incorrect synchronising operation, out-of-step protection operation, or network separation while the generator remains excited.

The out-of-phase condition between a generator and a power system can produce severe recovery-voltage stress even when the current is not as high as the maximum short-circuit current.

Generator-to-system out-of-phase recovery voltage

For a generator connected to the system through a step-up transformer, the recovery voltage is associated with the disappearance of the voltage drop across the generator, transformer and system reactances after current interruption. A simplified expression is:

\[ U_\text{RV} = I_\text{oop}\,k_\text{pp}\left( X''_d + X_\text{tr} + X_\text{sys} \right) \]

\(U_\text{RV}\): recovery voltage
\(I_\text{oop}\): out-of-phase current
\(k_\text{pp}\): overall first-pole-to-clear factor
\(X''_d\): generator subtransient reactance
\(X_\text{tr}\): transformer short-circuit reactance
\(X_\text{sys}\): equivalent system reactance

In most generator applications, a large part of the voltage drop is across the generator reactance and the transformer reactance; the system reactance may be smaller, especially when the grid is strong. The TRV waveform may contain components from both sides of the circuit-breaker, and these components may have different natural frequencies, so their peaks may not occur at exactly the same time.

Do not transfer the 25% rule to generator circuit-breakers

For generator circuit-breakers, the out-of-phase duty should be assessed using IEC/IEEE 62271-37-013 and the actual generator–transformer–system parameters. The general 25% value used for general-purpose circuit-breakers should not be used as a substitute for a generator-specific assessment.

Section 11

Switching between two power systems

Out-of-phase switching between two power systems typically occurs when two grid areas lose synchronism. This can happen during large disturbances, power imbalance, reactive-power imbalance, heavy loading, load rejection, protection maloperation, system restoration, or separation of a weak radial system.

In this case, the equilibrium point may be located on an overhead line, cable or interconnection between the two systems. The circuit-breaker that separates the systems may see travelling-wave effects, particularly if the separation point is on a transmission line. The initial rate of rise of recovery voltage may then be approximated by:

\[ \left.\frac{du}{dt}\right|_{t=0^{+}} = Z_\text{eq}\,\left.\frac{di_\text{oop}}{dt}\right|_{t=0^{-}} \]

\(\left.du/dt\right|_{t=0^{+}}\): initial rate of rise of recovery voltage, just after current zero
\(Z_\text{eq}\): equivalent surge impedance seen by the circuit-breaker
\(\left.di_\text{oop}/dt\right|_{t=0^{-}}\): rate of change of the out-of-phase current, just before current zero

For an overhead line, the equivalent surge impedance may be of the order of a few hundred ohms. The line-side TRV may have a triangular shape until travelling-wave reflections return from the remote end of the line.

Not only a lumped-reactance problem

This is why out-of-phase switching on long lines is not only a lumped-reactance problem. Travelling-wave behaviour and the configuration of the connected circuits at both ends can influence the TRV, so a transient (EMT) study is often the appropriate tool when the separation point is on a transmission line.

Section 12

Relationship with protection systems

Out-of-phase switching is normally not a desired operating condition. Protection and control systems are usually designed to avoid unnecessary interruption under severe out-of-phase conditions. Relevant schemes may include:

Synchro-check relays.
Out-of-step protection.
Power-swing blocking.
Pole-slip protection.
Generator loss-of-synchronism protection.
Controlled system-separation schemes.
Reclosing supervision.

However, protection schemes cannot eliminate all credible out-of-phase duties. Circuit-breakers in critical positions may still need an assigned out-of-phase switching capability, so protection design and circuit-breaker specification should be considered together rather than in isolation.

Section 13

Practical design and specification guidance

When preparing a circuit-breaker specification, the engineer should first decide whether out-of-phase switching is credible for the application. If it is, the duty should be included explicitly in the technical specification. The following questions help make that decision.

Specification checklist

Can this circuit-breaker connect or separate two live sources?
Is it used as a bus-coupler, bus-section or tie-line circuit-breaker?
Is it located between two systems that can lose synchronism?
Is it associated with a generator or power plant?
Could protection trip it during a power swing?
Could it be required to interrupt current during system restoration?
Could it close or trip under mis-synchronisation?
Is the connected system effectively earthed or non-effectively earthed?
Is the required out-of-phase current 25% of rated short-circuit current, or is a project-specific value required?
Is the TRV duty covered by the circuit-breaker type-test evidence?

Common mistakes

Common out-of-phase specification mistakes

Assuming a general-purpose circuit-breaker has out-of-phase capability without checking the specification, nameplate or type-test evidence.
Treating out-of-phase as a high-current duty — it is primarily a recovery-voltage (TRV) duty.
Checking only the current capability and not the associated TRV peak, RRRV and recovery voltage.
Placing the rating definition in IEEE C37.09 (test procedures) instead of IEEE C37.04 (ratings).
Using the general circuit-breaker standard for a generator circuit-breaker instead of IEC/IEEE 62271-37-013.
Ignoring the difference between effectively and non-effectively earthed neutral (the 2.0 versus 2.5 first-pole-to-clear factor).
Treating the 25% value as a guaranteed capability rather than an assigned value that still needs type-test evidence.
Forgetting travelling-wave / line-side TRV effects when the separation point is on a long transmission line.

Suggested report wording

Example report wording

“Out-of-phase switching refers to the interruption of current between two energised parts of a three-phase power system that are not in synchronism. It may occur during loss of synchronism between network areas, system separation, mis-synchronisation, generator instability or restoration switching. Although the out-of-phase current is normally lower than the rated short-circuit breaking current, the recovery voltage across the circuit-breaker can be severe because active sources remain connected on both sides of the circuit-breaker. For general high-voltage AC circuit-breakers above 1 kV, the out-of-phase switching duty should be assessed with reference to IEC 62271-100 or IEEE C37.04, depending on the project standard basis; IEEE C37.09 should be referenced for the corresponding test procedures, and for generator circuit-breakers IEC/IEEE 62271-37-013 should be used. Where an out-of-phase switching rating is assigned for a general-purpose circuit-breaker, a commonly specified value is 25% of the rated short-circuit breaking current unless otherwise stated by the applicable standard edition or project specification. The associated recovery-voltage duty shall also be checked, because out-of-phase switching is generally more critical for TRV peak than for current magnitude.”

Suggested specification clause

Example specification clause

“The circuit-breaker shall be suitable for out-of-phase switching duty where required by the application. The assigned out-of-phase breaking current shall be stated by the manufacturer and supported by type-test evidence in accordance with the applicable standard. For IEC-based specifications, the applicable reference shall be IEC 62271-100. For IEEE-based specifications, the rating basis shall be IEEE C37.04 and the corresponding test procedure shall be IEEE C37.09. Generator circuit-breakers shall be assessed in accordance with IEC/IEEE 62271-37-013. Unless otherwise specified by the project requirements, the assigned out-of-phase breaking current may be taken as 25% of the rated short-circuit breaking current for general circuit-breaker applications. The associated power-frequency recovery voltage, TRV peak and rate of rise of recovery voltage shall be confirmed against the circuit-breaker rating and type-test evidence.”

Section 14

Short engineering summary

Out-of-phase switching occurs when a circuit-breaker opens between two live systems that are not synchronised.
It is not normally a mandatory rating for every general-purpose circuit-breaker.
The usual assigned out-of-phase current for general circuit-breakers is 25% of the rated short-circuit breaking current unless otherwise specified.
The duty is severe mainly because of recovery voltage, not because of maximum current.
For general AC high-voltage circuit-breakers, use IEC 62271-100 or IEEE C37.04 / IEEE C37.09.
For generator circuit-breakers, use IEC/IEEE 62271-37-013.
Always check both current capability and TRV capability.

Key message

Out-of-phase switching is a moderate-current, high-recovery-voltage duty between two live, unsynchronised systems. It is an assigned capability — specify it explicitly where credible and require type-test evidence. A circuit-breaker may have a sufficient current rating but still be unsuitable if the out-of-phase TRV duty exceeds its tested capability, so always confirm both the current and the recovery-voltage (TRV) capability against the applicable standard edition.

Book cover of Switching in Electrical Transmission and Distribution Systems by Smeets, van der Sluis, Kapetanovic, Peelo and Janssen (Wiley)

Reference & further reading Switching in Electrical Transmission and Distribution Systems René Smeets, Lou van der Sluis, Mirsad Kapetanovic, David F. Peelo, Anton Janssen — Wiley

Part Four Reading now

Out-of-Phase Switching Duty

A moderate-current, high-recovery-voltage duty between two live, unsynchronised systems — the out-of-phase current and recovery voltage, factors and IEC / IEEE references.

Series progress 4 of 5

Part Five

Late Breakdown, Re-Ignition, Restrike & NSDD

Why a voltage collapse is not automatically a restrike — re-ignition, restrike, late breakdown and NSDD distinguished by the current that follows, for capacitive switching.